In reply to Grobbo:
I used to use a cheapish Haglof one which was quite broad and fitted my ME down bag well inside. But I must have bought it 15 years ago so not much help I'm afraid. I guess going to Decathlon or Go Outdoors and just looking at how different models vary in shape would be your best bet.
If you can find a suitable bag, I suspect you'll find the system works rather well. My old ME bag was rated to I think -7 and by putting the basic Haglof bag over it, I could keep warm down into the low negative teens. Particularly when car camping, it got me through a good few Finnish winters.